Gun barrel straightness testing apparatus



June 17, 1947. DU FONT 2,422,294

GUN BARREL STRAIGHTNESS TESTING APPARATUS Filed May 25, 1945 Fig. 1

INVENTOR ATTORN EY Patented June 17, 1947 GUN BARREL STRAIGHTNESS TESTING APPARATUS Francis G. du Pont, Utioa, N. Y., assignor to Remington Arms Company, Inc., Bridgeport, Conn., a corporation of Delaware Application May 23, 1945, Serial No. 595,444

Claims.

This invention relates to optical means for testing the bores of gun barrels and the like for straightness.

In the manufacture of small arms, such as callber .22 and caliber .30 rifles, the straightness of the barrels has been determined for a number of years by directing the bore toward a lighted glass and sighting through the bore. Then by experience the operator can ascertain whether or not the barrel is straight, but the actual amount of deviation can only be estimated. Such visual determination obviously requires considerable training on the part of the operators and is slow and tedious. One of the objects of this invention is to provide a simple optical means for testing the straightness of a gun which allows accurate measurement of the location and magnitude of the deviations.

Other objects of the invention will become apparent from the following description, which description and illustrations are merely exemplary and are not intended to limit the invention to the form shown and described.

In the drawings:

Fig. 1 is an elevational view partially in cross section showing a gun barrel with the optical assembly of the present invention in place.

Fig. 2 is a side elevation partially in cross-section showing the optical assembly of the muzzle cross hair and the intermediate lens.

Fig. 3 is an end elevational view of the muzzle cross hair piece.

Fig. 4 is a diagrammatic showing of the paths of the rays of light as conducted by the optical assembly.

The length of gun barrels is such that it is difiicult to maintain the bore straight throughout the machining operations and the occasional tendency to deviate from a straight condition may be caused by various reasons. It is desirable that the barrel be tested so as to eliminate barrels having crooked bores or segregate such barrels for straightening operations.

Referring to Fig. 1, a gun barre1 is shown at Ill having the usual threads H at the breech end. The barrel may be supported in two conventional V-blocks l2 and I 3. The muzzle cross hair assembly is seen at M. Said assembly has a tube l5 with a pilot [6 for closely engaging the bore of the gun. The tube terminates in a flange I! which is clamped between plates [8 and I9 by bolts 20. A slot 2| is provided in plate [9 to accommodate a glass 22 containing the cross hairs 23, the intersection of the cross hairs falling on the axis of the pilot IS. A lens assembly 24 is arranged to be placed in the barrel and positioned so that the cross hairs 23 of the muzzle plug will be located at one of the conjugate focal points thereof. The assembly may take the form of a tube 25 of slightly less than bore diameter having a suitable plano-convex type lens 26 therein, the optical axis of the lens preferably being the same as or parallel to the axis of the tube. The lens is so positioned that the other conjugate focal point will fall on the reticule or screen of the filar micrometer eyepiece shown diagrammatically at 2:1. This eyepiece has a vertically movable reticule contained in the housing 28 which is movable by means of the micrometer screw 29. One type of such filar micrometer is shown on page 24 of Bausch & Lomb Catalog E-225 entitled Metallographic equipment and accessories.

As seen in Fig. 2, the tube 25 may have a spring 30 so as to firmly hold the tube in contact with the wall of the bore of the gun. The spring will prevent lost motion which might allow the optical center of the lens to rest at a different distance from the wall of the bore. Tube 25 may be connected to the plug l5 by means of a spring steel connector 3|. There also may be a steadying plug 32 between the muzzle plug and lens assembly which serves to support the flexible connector 3| against bucklin and thus insures that the lens will always be placed at the same distance from the reticule plate. It also is evident that the lens assembly 24 may be entirely separate from the muzzle plug.

Tube 33 is fastened to the filar micrometer and one end of said tube is allowed to rest in the V- block l3 and, if desired, it may be clamped thereto by clamp 34.

Referring to Fig. 4, the cross hair is indicated at 23 and the lens at 26. The reticule plane of the micrometer is represented by line 31. The assembly is placed in the end of the gun barrel and, as shown in full lines, the rays of light 38 will cause a real image to appear at 39 in the plane of the micrometer reticule. The micrometer screw may be then revolved so as to bring the cross hairs or indicia on the reticule of the micrometer into coincidence with the image, the reading of the micrometer being noted. The optical assembly may then be removed from the barrel and the barrel rotated 180 in the V-blocks after which the optical assembly is then reinserted into the barrel, in its former longitudinal position and orientation relative to the micrometer.

Assuming that the threads I! are concentric with the bore of the gun and that the barrel is straight, the image will appear at the same point on the screen 31. However, if the barrel is crooked at the point where the lens assembly 24 is located, thus causing the optical center of the lens to be shifted laterally or vertically, an

tricity of the cross hairs and the object lens and the difference between the diameters of the lens support tube and the barrel are cancelled. Onehalf of the difference between the micrometer readings will be the deviation measured in a vertical plane of an imaginary line extended through the cross hair intersection and the optical center of the object lens 26 from the axis of rotation of the barrel determined by the V-blocks.

Another set of readings may be taken in positions spaced 90 from those of the previous test and the two distances thus found may be added vectorially. A series of determinations may be made with object lenses of different focal lengths placed at different distances from the end of the barrel and thereby the contour of the barrel may be determined for its full length. If the breech end of the bore and the outside diameter of the threads I I are not truly concentric, the deviation determined above will be in error. A suitable correction for such a lack of exterior concentricity may be readily determined and may be applied to the readings obtained by the optical means.

As one example, in determining the contour of a caliber .30 rifle barrel, one lens having a 4.40 inch focal length may be positioned approximately six inches from the muzzle. Standard lens formulae will show that the image will appear at a distance of approximately 16.5 inches from the lens, at which position the micrometer reticule should be placed. Readings taken as outlined in the preceding paragraphs will conclusively determine the relationship of the center of the bore at a point six inches from the muzzle to a line passing from the center of the bore at the muzzle to a point at the breech of the barrell on the axis of rotation of the barrel on the V-blocks. A set of six lenses varying in focal length from 1.86 inches to 6.70 inches would be sufficient to chart a 24 inch barrel at 2 inch intervals provided the barrel is once reversed in the V-blocks and a series of siX readings taken from each end.

What is claimed is:

l. A gun barrel straightness testing apparatus comprising a bore plug with an index mark located on the axis of the plug, said bore plug being removably inserted in an end of the bore of said gun barrel in centered relation thereto; means to illuminate said index mark; an optical micrometer located at the end of said barrel opposite to the bore plug, said micrometer having a transversely adjustable index; means to measure the position of said index; and a holder removably supporting an object lens in said bore, the lens being so positioned that its conjugated foci fall at the index mark and at said adjustable index.

2. A gun barrel straightness testing apparatus comprising a bore plug with a set of cross hairs, said bore plug being removably inserted in an end of the bore of said gun barrel with the intersection of the cross hairs on the axis thereof; means to illuminate said cross hairs; an eyepiece lens assembly located at the end of said barrel opposite to the bore plug, said eyepiece lens having a transversely adjustable index at its focus; means to measure the position of said index; and a tube carrying a converging lens removably inserted in said bore in predetermined orientation relative to the cross hairs and to the eyepiece lens assembly, the converging lens being so positioned longitudinally of the bore that its conjugate focal points fall in the plane of the cross hairs and in the plane of the adjustable index.

3. A gun barrel straightness testing apparatus comprising a bore plug with a set of cross hairs, said bore plug being removably inserted in an end of the bore of said gun barrel with the intersection of the cross hairs on the axis thereof; means to illuminate said cross hairs; a tube removably inserted in said barrel having a lens therein; means connecting said bore plug and said tube and maintaining them in the same orientation and at a spacing such that one of the conjugate foci of the lens is in the plane of the cross hairs; an optical micrometer located at the end of said barrel opposite to the bore plug and focused on the other conjugate focal point of said lens, said optical micrometer including a, transversely adjustable index in its focal plane; and means to measure the position of said index.

4. A gun barrel straightness testing apparatus including a set of V-blocks rotatably supporting the ends of said barrel; a bore plug with a set of cross hairs, said bore plug being removably inserted in an end of the bore of said gun barrel to position the intersection of the cross hairs on the axis thereof; means to illuminate said cross hairs; an eyepiece lens assembly located on one of the V-blocks at the end of said barrel opposite to the bore plug, said eyepiece lens having a transversely adjustable index in its focal plane; means to measure the position of said index; and a tube carrying a converging lens removably inserted in said bore in predetermined orientation relative to the cross hairs and the eyepiece lens assembly, the tube being so positioned longitudinally of the bore that the conjugate focal points of the converging lens fall in the plane of the cross hairs and in the plane of the adjustable index.

5. A gun barrel straightness testing apparatus including a set of V-blocks rotatably supporting the ends of said barrel; a bore plug with a set of cross hairs removably inserted in an end of the bore of said gun barrel with the intersection of the cross hairs on the axis thereof; means to illuminate said cross hairs; a tube removably inserted in said barrel and having a lens therein; flexible means connecting said bore plug and said tube maintaining them in predetermined orientation and at a spacing such that one of the conjugate foci of said lens is in the plane of the cross hairs; an eyepiece assembly including an ocular lens, a transversely adjustable index at the focus of said ocular lens; and means to measure the position of said index, said eyepiece assembly being located on one of the V-blocks at the end of said barrel opposite to the bore plug and in a position in which the other conjugate focal point of said first mentioned lens falls in the plane of the adjustable index.

FRANCIS G. DU PONT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,079,791 Cook May 11, 1937 FOREIGN PATENTS Number Country Date 310,142 Germany Nov. 6, 1919 

